In semiconductor fabrication processes, for purposes of preparing an ultra-clean semiconductor wafer surface, a wafer surface typically undergoes a cleaning process using a wide variety of chemical liquids or gases according to different process requirements. Currently, most of the cleaning processes have still been based on traditional RCA cleaning recipes (referred to as “SC1 and SC2 cleaning solutions”), as well as cleaning solutions formed of a mixture of sulfuric acid and hydrogen peroxide (referred to as “SPM”). The cleaning solution of the mixture of sulfuric acid and hydrogen peroxide is employed not only to remove organic contaminants and photoresists on the wafer surface, but also to generate a thin oxide layer on the wafer surface. The No. 1 cleaning solution (SC1), formed of a mixture of ammonium hydroxide and hydrogen peroxide, is used to remove particles, some organic compounds and some metals on the wafer surface. The No. 2 cleaning solution (SC2), formed of a mixture of hydrochloric acid and hydrogen peroxide, is used to remove metal ions on the wafer surface. RCA cleaning recipes contain hydrogen peroxide which is used in oxidation reactions, whereas the SPM cleaning process consumes a large quantity of concentrated sulfuric acid. Both are to be followed by rising with a large quantity of ultra-pure water. Regardless RCA recipes or SPM, it is environmentally unsound and poses a potential threat to the safety and health of operating personnel involved. Therefore, it is essential to look for a safe, economical and environment-friendly alternative cleaning process and recipe of which a cleaning effect is comparable to or better than that of SPM.
It is well known that ozone gas possesses extremely strong oxidizability. That is, ozone gas is capable of having oxidation-reduction chemical reactions with a plurality of substances, such as organics and silicon. In the oxidation-reduction reactions, ozone gas is reduced to oxygen, which is harmless to humans and environment. Therefore, ozone gas, as well as an ozone aqueous solution formed of a mixture of ozone gas and water, constitute safe, economical and environment-friendly cleaning process and recipe. Although ozone aqueous solution has been studied for the purpose of removal of organic contaminants and photoresists, it has not been widely used. The main obstacle resides in that a concentration of the ozone in chemical reactions is difficult to ensure or control, and thus cannot fulfill processing requirements.
Normally, the dissolved concentration of ozone gas in water follows Henry's law. Namely, a concentration of ozone in liquid phase is proportional to a partial pressure of ozone in gas phase. Therefore, it is necessary to increase the partial pressure of ozone in gas phase in order to obtain an ozone aqueous solution having a high ozone concentration. Existing cleaning equipment, whether based on immersion or spray techniques, prepares an ozone aqueous solution of higher concentration by premixing ozone and water, before treating the wafer surface with the prepared ozone aqueous solution using either a soaking reservoir containing the prepared ozone aqueous solution or a spraying mechanism spraying the prepared ozone aqueous solution onto the wafer surface. Regardless, due to a rather open environment of the existing cleaning equipment, it is difficult to maintain a high ozone concentration in gas phase. Since the partial pressure of ozone in gas phase is relatively low, once the ozone aqueous solution is transferred into the soaking reservoir or sprayed onto the wafer surface, the ozone in the water would immediately start to cross an interface of liquid and gas and escape into the gas phase. Therefore, the ozone concentration in the ozone aqueous solution will reduce rapidly and continuously, thereby negatively influencing the processing effect.